1. Field of the Invention
The invention relates to the field of radar absorbing coatings and, in particular, to an improved coating incorporating iron particles.
2. Description of Related Art
Typical radar absorbing material (RAM) coatings incorporate iron particles in a resin that is either spray painted on the surface of the vehicle or applied thereon in the form of decals. The iron particles can also be incorporated into a ceramic matrix material. For example, U.S. Pat. Nos. 5,164,242 "Electromagnetic Wave Attenuating And Deicing Structure" by S. D. Webster, et. al, and 5,338,617 "Radio Frequency Absorbing Shield And Method" by D. M. Workinger, et. al. discloses the use of Carbonyl iron in a resin matrix, while U.S. Pat. No. 5,085,931 "Microwave Absorber Employing Acicular Magnetic Metallic Filaments" by C. E. Boyer, et al. discloses the use of filaments having an average length of 10 microns and diameters of about 0.1 micron. for use in an absorber. U.S. Pat. No. 4,003,840 "Microwave Absorber" by K. Iishino, et. al. suggests 1.65 mm ferrite powder in an organic high molecular compound; for example 0.2 to 0.9 part by volume ferrite powder and 0.8 to 0.1 organic high molecular compound. U.S. Pat. No. 3,568,195 "Electromagnetic Wave Attenuating Device" by L. Wesch, et. al. discloses an absorber comprising an outer radar wave attenuating layer that can incorporate iron powders and a non-metallic backing sheet.
In a good light weight specular RAM coating high attenuation level and broad frequency range are important. However, with such coatings peak attenuation band width decreases with decreasing frequency and causes attenuation at frequencies other than the peak attenuation frequencies to be less than 5 dB.
One common technique to improve the broad band response of a specular RAM is to use multiple coatings separated by some kind of a band pass filter. For example in U.S. Pat. Nos. 5,169,713 "High Frequency Electromagnetic Radiation Absorbent Coating Comprising A Binder And Chips From A Laminate Of Alternating Amorphous Magnetic Films And Electrically Insulating" by P. Kmurdjian. Kmurdjian discloses the use of multiple layers having a thickness in the 2-5 nanometer range, with each layer including an amorphous magnetic film and an insulating film of 1-5 electrically insulating material. In U.S. Pat. No. 4,581,284 "Fiber Compound Material" by D. Ggumbh a structure is disclosed made of fiber plies impregnated with a radar absorbing compounds in a concentration varying from the exterior to the interior side. U.S. Pat. No. 5,147,718 "Radar Absorber" by S. A Papoulias, et. al. discloses the use of a multi-layer absorber having a first layer with 4 to 5 micron carbonyl iron powder and a second layer with 0.5 to 1.5 micron powder. The inventor claims that such an absorber provides a relatively high radar attenuation magnitude over a selected broad band frequency range. U.S. Pat. No. 4,024,318 "Metal-Filled Plastic Material" by E. O. Forster, et. al. discloses the use of a multi-layer material wherein the first layer is filed with metal particles in a resin matrix and a second contains metal oxides in a resin matrix. However, such multiple layer absorbers have weak shear planes between layers, are expensive and, additionally, create field maintenance problems. A problem of both single and multiple coating is their high unit weight.
The performance of these coatings, particularly those using spherical particles, is dependent upon how closely the spheres are packed together. Thus the most efficient coating would be one approaching the density of solid iron with a minimum amount of resin included to electrically insulate the particles from one another. That is, the attenuation efficiency increases faster than the weight, so that a thinner coating with the same attenuation, can be used, providing an overall weight savings. Unfortunately, the particles, when produced, are of non-uniform diameter and not necessarily uniformly round. Even with filtering for size or centrifugal particle separation methods, a Gaussian distribution about the selected diameter occurs. Thus the best packing densities are around 4.5 grams per cubic centimeter for 5 micron diameter particles, when 5.7 grams per cubic centimeter could be obtained if all the particles were of exactly one diameter.
Thus it is a primary object of the subject invention to provide an improved radar absorbing material.
It is another primary object of the subject invention to provide an improved radar absorbing material that is lighter in weight than conventional absorbers having equal performance.
It is a further object of the subject invention to provide an improved single layer radar absorbing material that is lighter in weight than conventional absorbers having equal performance.
It is a still further object of the subject invention to provide an improved radar absorbing material that has a greater packing density when the spheres of magnetic material are distributed about a mean diameter.